Business Segments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Reimbursement Revenues	$ 1,918.3	$ 1,740.4	$ 3,777.3	$ 3,404.5
Segment revenue:
Revenue	4,266.5	3,903.7	8,087.1	7,458.9
Revenue Before Reimbursements	2,348.2	2,163.3	4,309.8	4,054.4
Operating expenses:
Depreciation and amortization	45.5	46.3	92.0	88.4
Operating Income (Loss)	150.1	149.8	175.5	203.6
Equity in earnings (losses)	10.2	10.2	15.2	23.8
Segment Reconciling Items:
Revenue	4,266.5	3,903.7	8,087.1	7,458.9
Total segment operating expenses before restructuring and acquisition charges	2,172.4	2,024.6	4,090.0	3,861.2
Cost of Reimbursable Expenses	1,918.3	1,740.4	3,777.3	3,404.5
Segment Operating Expenses before Restructuring and acquisition	4,090.7	3,765.0	7,867.3	7,265.7
Segment Reporting Information Operating Income Before Restructuring Charges	175.8	138.7	219.8	193.2
Restructuring and acquisition charges	25.7	(11.1)	44.3	(10.4)
Operating Income (Loss)	150.1	149.8	175.5	203.6
Assets	10,580.9		10,580.9		$ 10,025.5
Reportable Subsegments [Member]
Segment revenue:
Real Estate Revenues, Net	1,630.0	1,493.5	2,949.1	2,775.0
Operating expenses:
Gross contract vendor and subcontractor costs reimbursement	713.4	668.5	1,356.0	1,275.4
Reportable Subsegments [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	1,403.1	1,226.4	2,754.5	2,408.1
Segment revenue:
Revenue	2,463.6	2,152.5	4,713.7	4,093.5
Revenue Before Reimbursements	1,060.5	926.1	1,959.2	1,685.4
Derivative, Gain (Loss) on Derivative, Net	4.8	1.3	4.7	4.0
Real Estate Revenues, Net	863.9	768.2	1,575.0	1,391.7
Operating expenses:
Compensation, operating and administrative expenses	913.7	796.9	1,724.5	1,485.6
Depreciation and amortization	26.3	28.3	53.6	52.6
Operating Expenses, Excluding Reimbursed Expenses	940.0	825.2	1,778.1	1,538.2
Gross contract vendor and subcontractor costs reimbursement	191.8	156.6	379.5	289.7
Total fee based segment operating expenses	748.2	668.6	1,398.6	1,248.5
General Contractor Cost	(191.8)	(156.6)	(379.5)	(289.7)
Operating Income (Loss)	120.5	100.9	181.1	147.2
Equity in earnings (losses)	0.4	0.4	0.1	0.5
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	120.9	101.3	181.2	147.7
Segment Reconciling Items:
Revenue	2,463.6	2,152.5	4,713.7	4,093.5
Operating Income (Loss)	120.5	100.9	181.1	147.2
Reportable Subsegments [Member] \| EMEA [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	153.7	153.0	318.3	309.0
Segment revenue:
Revenue	818.3	846.6	1,541.7	1,630.2
Revenue Before Reimbursements	664.6	693.6	1,223.4	1,321.2
Real Estate Revenues, Net	379.9	388.3	696.0	738.7
Operating expenses:
Compensation, operating and administrative expenses	654.6	683.5	1,231.5	1,319.7
Depreciation and amortization	11.3	11.5	22.6	22.9
Operating Expenses, Excluding Reimbursed Expenses	665.9	695.0	1,254.1	1,342.6
Gross contract vendor and subcontractor costs reimbursement	284.7	305.3	527.4	582.5
Total fee based segment operating expenses	381.2	389.7	726.7	760.1
General Contractor Cost	(284.7)	(305.3)	(527.4)	(582.5)
Operating Income (Loss)	(1.3)	(1.4)	(30.7)	(21.4)
Equity in earnings (losses)	(1.1)	0.0	(1.0)	0.0
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	(2.4)	(1.4)	(31.7)	(21.4)
Segment Reconciling Items:
Revenue	818.3	846.6	1,541.7	1,630.2
Operating Income (Loss)	(1.3)	(1.4)	(30.7)	(21.4)
Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	359.6	356.2	700.7	677.8
Segment revenue:
Revenue	855.2	812.9	1,603.9	1,524.2
Revenue Before Reimbursements	495.6	456.7	903.2	846.4
Real Estate Revenues, Net	262.9	251.3	461.1	445.7
Operating expenses:
Compensation, operating and administrative expenses	462.6	427.5	863.1	812.5
Depreciation and amortization	6.4	5.8	12.8	11.4
Operating Expenses, Excluding Reimbursed Expenses	469.0	433.3	875.9	823.9
Gross contract vendor and subcontractor costs reimbursement	232.7	205.4	442.1	400.7
Total fee based segment operating expenses	236.3	227.9	433.8	423.2
General Contractor Cost	(232.7)	(205.4)	(442.1)	(400.7)
Operating Income (Loss)	26.6	23.4	27.3	22.5
Equity in earnings (losses)	0.4	0.7	0.7	1.0
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	27.0	24.1	28.0	23.5
Segment Reconciling Items:
Revenue	855.2	812.9	1,603.9	1,524.2
Operating Income (Loss)	26.6	23.4	27.3	22.5
Investment Management [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	1.9	4.8	3.8	9.6
Segment revenue:
Revenue	129.4	91.7	227.8	211.0
Revenue Before Reimbursements	127.5	86.9	224.0	201.4
Real Estate Revenues, Net	123.3	85.7	217.0	198.9
Operating expenses:
Compensation, operating and administrative expenses	96.0	70.4	178.9	155.0
Depreciation and amortization	1.5	0.7	3.0	1.5
Operating Expenses, Excluding Reimbursed Expenses	97.5	71.1	181.9	156.5
Gross contract vendor and subcontractor costs reimbursement	4.2	1.2	7.0	2.5
Total fee based segment operating expenses	93.3	69.9	174.9	154.0
General Contractor Cost	(4.2)	(1.2)	(7.0)	(2.5)
Operating Income (Loss)	30.0	15.8	42.1	44.9
Equity in earnings (losses)	10.5	9.1	15.4	22.3
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	40.5	24.9	57.5	67.2
Segment Reconciling Items:
Revenue	129.4	91.7	227.8	211.0
Operating Income (Loss)	30.0	15.8	42.1	44.9
Leasing [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	491.8	422.9	881.6	727.4
Segment Reconciling Items:
Revenue	491.8	422.9	881.6	727.4
Leasing [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	65.9	72.9	118.1	131.8
Segment Reconciling Items:
Revenue	65.9	72.9	118.1	131.8
Leasing [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	66.2	58.1	102.1	95.9
Segment Reconciling Items:
Revenue	66.2	58.1	102.1	95.9
Capital Markets [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	129.8	117.8	229.9	228.8
Segment Reconciling Items:
Revenue	129.8	117.8	229.9	228.8
Capital Markets [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	78.5	89.9	142.5	179.2
Segment Reconciling Items:
Revenue	78.5	89.9	142.5	179.2
Capital Markets [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	48.2	50.3	77.6	82.7
Segment Reconciling Items:
Revenue	48.2	50.3	77.6	82.7
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	1,370.9	1,258.4	2,732.9	2,441.9
Segment Reconciling Items:
Revenue	1,370.9	1,258.4	2,732.9	2,441.9
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	380.6	392.3	749.9	742.6
Segment Reconciling Items:
Revenue	380.6	392.3	749.9	742.6
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	553.4	541.6	1,091.2	1,061.4
Segment Reconciling Items:
Revenue	553.4	541.6	1,091.2	1,061.4
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	376.8	274.5	685.5	544.1
Segment Reconciling Items:
Revenue	376.8	274.5	685.5	544.1
Project & Development Services [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	219.2	220.8	399.7	443.4
Segment Reconciling Items:
Revenue	219.2	220.8	399.7	443.4
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	137.3	114.1	248.2	201.1
Segment Reconciling Items:
Revenue	137.3	114.1	248.2	201.1
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	94.3	78.9	183.8	151.3
Segment Reconciling Items:
Revenue	94.3	78.9	183.8	151.3
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	74.1	70.7	131.5	133.2
Segment Reconciling Items:
Revenue	74.1	70.7	131.5	133.2
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	50.1	48.8	84.8	83.1
Segment Reconciling Items:
Revenue	50.1	48.8	84.8	83.1
Advisory Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	79.5	67.1	157.1	137.0
Segment Reconciling Items:
Revenue	79.5	67.1	157.1	137.0
Transaction Fees & Other [Member] \| Investment Management [Member]
Segment revenue:
Revenue	16.3	6.7	29.5	23.4
Segment Reconciling Items:
Revenue	16.3	6.7	29.5	23.4
Incentive Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	33.6	17.9	41.2	50.6
Segment Reconciling Items:
Revenue	$ 33.6	$ 17.9	$ 41.2	$ 50.6

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